I am in a minority here and I’ve always seen the overwhelming majority to suggest going to London or Windsor but as another poster pointed out, time soon dwindles down. In my opinion the idea of heading into London or Windsor, unless you have a specific plan or location, just feels tiring thinking about it. London nor Windsor (given it’s attraction) is relaxing unless you’re dining out in a nice restaurant/hotel but that’s not ideal during transit. Walking around a museum or park might be nice for a change of scenery but it’s still a busy place and you have to get there.

Sitting in the place (airport lounges) where you know you will be departing in a few hours, ability to sit, read (learn/work), eat and drink, to me feels a bit more relaxing than dealing with the complexities London can throw at you, transport delays, busy areas, lots of walking around just to get somewhere. Some people prefer a good exercise and scenery, so London or Windsor may well appeal. But overall its a personal choice. London isn’t the be it and end all. It’s just another mega city with lots of nice landmarks, millions of people and (suspect) transport. Give me a small beach village or a mountain escape anytime over London, Dubai, NY etc. or just staring out at the LHR aprons watching the wonders of aviation defy gravity.

Another alternative is a hotel. Hilton T5 was a nice place, I’m unsure if the pool/spa area ever reopened after covid.